1. FIELD OF THE INVENTION
This invention relates to plastic lens having a high refractive index of not less than 1.55 and excellent properties in visible light transmission, processability, impact resistance, and dyeability.
2. DESCRIPTION OF THE PRIOR ART
Plastic lenses have become widely used in optical instruments, since they are lighter in weight, less fragile and more readily colored in comparison with inorganic glass lenses. Especially, in the case of eyeglass lenses, light-weight lenses are desirable, because the total weight of a pair of lenses is important from both physiologic and optical viewpoints. This is one of the main reasons for the recent widespreading tendency of plastic lenses. The leading resin used for eyeglass lenses is a polymer of diethylene glycol bis (allylcarbonate) (commonly called CR-39, Trademark for PPG Industries Co.). Since the specific gravity of the above polymer (1.32) is smaller than that of inorganic glass lenses (2.54), it is possible to reduce to a great extent the weight of lenses when this polymer is used. The above polymer, however, has a refractive index of 1.49 to 1.50 which is smaller than that (n.sub.D =1.52) of inorganic glass lenses. As a consequence, it is necessary to increase the central thickness, peripheral thickness and curvature of the lenses made of this polymer, as compared with inorganic glass lenses. Accordingly, development of a plastic lens having a higher refractive index has been eagerly awaited.
Polycarbonate (n.sub.D =1.58) and polystyrene (n.sub.D =1.60) are known popularly as polymers having high refractive indices. Since both of these polymers are linear and thermoplastic, they are not suitable for castpolymerization method and hence unfit for production of articles of varied types, for example, production of eyeglass lenses. Moreover, the use of these polymers in optical field is limited to comparatively small areas owing to their disadvantages such as unsatisfactory grinding processability, low scratch resistance due to insufficient surface hardness, low resistance to organic solvents, and low heat resistance.
The proposed methods for providing three-dimensionally crosslinked plastic lens materials having high refractive indices include those which employ as major constituents (1) diacrylate or dimethacrylate derived from bisphenol-A [Japanese patent application "Kokai" (Laid-open) No. 13747/80], (2) diacrylate or dimethacrylate derived from halogenated bisphenol-A [Japanese patent application "Kokai" (Laid-open) No. 104,901/82], (3) a combination of a halogenated styrene monomer and a polyfunctional methacrylate [Japanese patent application "Kokai" (Laid-open) Nos. 104,101/82, 28,118/82, and 28,116/82], and (4) a diallyl phthalate monomer [Japanese patent application "Kokai" (Laid-open) Nos. 212,401/82 and 15,513/83].
However, the method (1) provides with difficulty three-dimensionally crosslinked plastics having a refractive index of not less than 1.55. The methods (2), (3) and (4) provide three-dimensionally crosslinked plastics having a high refractive index of not less than 1.60, but these plastics have faults of their easy discoloration and unsatisfactory weatherability. The plastics obtained by the method (4) have not only the fault of discoloration but also a low transmittance. In general, methods using compounds containing a (meth)acryloyl group and styrene monomers are disadvantageous in that said compounds contain polymerization inhibitors and are difficult to alleviate discoloration of the cured products caused by said inhibitors during the castpolymerization.